Congenital kidney diseases are leading causes of chronic kidney disease in children. The application's broad long-term objectives are to understand molecular control of kidney development to make an impact on congenital kidney disease. Fibroblast growth factor receptors (Fgfrs) that signal through adapter proteins such as fibroblast growth factor receptor substrate 2 (Frs2) are critical for kidney development; however, global mouse gene targeting has not revealed the full impact of Fgfr or Frs2 signaling in fetal kidneys due to early lethality of the mutant embryos. Furthermore, conditional deletion of Fgfr1 and Fgfr2 in the metanephric mesenchyme (MM) with a Pax3cre line led to renal dysgenesis. To elucidate roles of Fgfr/Frs2 signaling in the MM at later stages of renal development, compound mutants were generated with 1. Deletion of Fgfr1 in increasingly restricted and later stages of MM development and 2. Global point mutations in the Frs2 binding site in Fgfr2. The mutant mice with Fgfr1 deletion in early MM with the Pax3cre line and point mutations in the Frs2 binding site in Fgfr2 (PFLR mice), escape the renal dysgenesis seen with Pax3cre deletion of Fgfr1 and Fgfr2; however, PFLR mice developed ureteric bud (UB) branching defects, premature nephron progenitor (NP) depletion, and progressive cystogenesis in both ureteric and nephron lineages (showing importance of Frs2 directed signaling downstream of Fgfr2). The ureteric defects in PFLR mice start with dilated and hyper- proliferative UB tips that appear to be linked to inappropriate Ret signaling. The mice also appear to have defects in ciliary structure, canonical Wnt signaling, and planar cell polarity (PCP) that each may contribute to the developmental and cystic defects. Another set of mice, generated with a Six2cre line (SFLR) that deletes Fgfr1 slightly later than PFLR and only in NPs (more restricted than PLFR) cause NP depletion and cysts from the nephron lineages, but have no apparent ureteric defects. SFLR NPs initially form normally, but appear to lose stem cell-like markers and undergo apoptosis. The third set of mice, generated with a Wnt4cre line (WFLR) that delete in renal vesicles later than SFLR have no apparent renal abnormalities. The hypothesis is that spatial and temporal regulation of Fgfr signaling through Frs2 in the metanephric mesenchyme is critical for both ureteric and nephrogenic lineage patterning during development. To test the hypothesis, the following Aims were generated: Aim 1. To characterize the non-autonomous ureteric and autonomous nephrogenic lineage patterning defects in the allelic series of mutant mice. Embryonic and post-natal kidneys will be assessed for structural and functional defects. Aim 2. To determine mechanisms by which Fgfr/Frs2 signaling in kidney mesenchyme leads to ureteric morphogenesis defects and cystogenesis. Roles of the Bmp4/Ret/Erk axis and ciliary, canonical Wnt and PCP defects will be assessed. Aim 3: To determine mechanisms by which Fgfr/Frs2 signaling in kidney mesenchyme leads to premature nephron progenitor depletion. Candidate targets of Fgfr/Frs2 signaling identified by unbiased assays will assessed in NP cell cultures.